The invention relates to a method of making a variable capacitor microphone and to a microphone made by the method.
Capacitor microphones in which a permanently polarized dielectric, or electret, is used to obviate the need for an external high-voltage supply are well known in the art. Because of their small size, relative insensitivity to temperature or humidity changes, and extended high-frequency response, they are especially suited for use in citizen's-band transceivers, military communications systems and the like. One such electric microphone is shown in Cote U.S. Pat. No. 4,443,666, owned by the assignee herein.
In the microphone shown in the above-identified patent, an insulating substrate carries a metal electrode which in turn supports an electret. The electret comprises a fluorocarbon polymer such as the one sold by E. I. du Pont de Nemours and Company under the trademark TEFLON. A diaphragm comprising a metallized polyester such as the one sold under the trademark MYLAR, disposed at a spacing from the electret, serves as a second electrode. Vibration of the second electrode in response to acoustic vibrations gives rise to a corresponding variation in the instantaneous displacement between the two electrodes, producing in turn a momentary variation in the potential difference between the two electrodes. This potential variation is applied to the gate of a field-effect transistor (FET) carried on the other side of the substrate, which in turn is coupled to an external amplification system.
In the fabrication process conventionally used, individual electrets of the desired shape are cut from a previously polarized sheet of TEFLON or other fluorocarbon polymer and bonded to the electrode carried by the substrate, following which the other elements of the electret microphone are assembled. It has been found that electrets made by this conventional process tend to discharge after a relatively short period of time, especially in moist environments. While the exact explanation of this premature discharge is not known, it is believed that the repeated handling of the electret following its initial formation is a contributing factor.
Another problem experienced with this conventional fabrication process is the tendency of the electret not to adhere to the electrode to which it is applied.
Still another problem experienced with the conventional fabrication process involves the polarization of the dielectric layer. One common method of polarizing the dielectric entails the use of a corona to ionize a gaseous region adjacent to the layer to generate charged particles. Although coronas generate the desired charged particles, they also undesirably generate ozone.